1. Field of Invention
This invention relates to processes for making ammonia. More specifically, this invention relates to processes for manufacturing ammonia on an industrial scale from a heterogeneous feedstock containing a time varying variety of different materials having different carbon contents and including, for example, solid or liquid carbon-containing waste materials.
2. Description of Related Art
Ammonia is produced on an industrial scale by the catalytic conversion of nitrogen and hydrogen at high pressures and temperatures. While the nitrogen typically comes from air, the hydrogen is usually produced by reacting carbon-containing feedstocks with steam (steam reforming) or oxygen (partial oxidation). For example, methane or natural gas can be catalytically steam reformed (CH.sub.4 +H.sub.2 O.fwdarw.CO+3H.sub.2). In contrast, coal, coke and naphtha or heavy oils, for example, can undergo reactions including partial oxidation (2C+O.sub.2 .fwdarw.2CO) and water gas reaction (C+H.sub.2 O.fwdarw.CO+H.sub.2). The steam reforming and partial oxidation reactions form a gas ("synthesis gas"), which usually contains primarily CO and H.sub.2, along with a small amount of CO.sub.2. The CO in the synthesis gas is combined with steam in the so-called "shift reaction" to form a shifted gas containing CO.sub.2 and H.sub.2 (CO+H.sub.2 O.fwdarw.CO.sub.2 +H.sub.2). After separating the carbon oxides and other undesired components in the shift gas from the H.sub.2, the H.sub.2 is combined with N.sub.2 to form ammonia.
To separate the carbon oxides in the shifted gas from the H.sub.2, a number of processes are conventionally used. For example, the bulk of the CO.sub.2 can be removed by absorption (i.e., scrubbing), using suitable physical solvents, such as methanol and esters of oligoethylene glycols, or chemical-type solvents, such as hot potassium carbonate and solutions of amines. Following the absorption treatment, the shifted gas mixture can be passed to a methanation reactor for conversion of carbon oxides to lower levels. The conventional methanation step is essentially the reverse of the steam reforming step, but carried out at lower temperature, wherein CO and CO.sub.2 are caused to react with hydrogen to form methane and water (CO+3H.sub.2 .fwdarw.CH.sub.4 +H.sub.2 O; CO.sub.2 +4H.sub.2 .fwdarw.CH.sub.4 +2H.sub.2 O).
Alternatively, carbon oxides in the shifted gas can be separated from the H.sub.2 used for ammonia synthesis through the use of pressure swing adsorption ("PSA") techniques. A PSA system is capable of selectively adsorbing CO.sub.2, CO, CH.sub.4 and other impurities from H.sub.2 and from a portion of any N.sub.2 present in the shifted gas mixture leaving the shift reactor. In most known PSA systems, the product gas is not discharged continuously, and therefore, a plurality of adsorbent beds are provided in parallel with one another to achieve a measure of continuity in the product output flow. In the PSA separation process of each adsorption bed, at least one selectable component of the feed gas mixture is adsorbed so that the gas discharged at the other end of the adsorption bed is the component-depleted product gas. Generally, such adsorption occurs at the highest pressure of the process, which is generally the input feed pressure. This high pressure portion of the separation process cycle is followed by a depressurization portion of the cycle wherein the gas within the adsorption bed is reversed in its direction of flow and released at the inlet end of the adsorption bed. The gas which is thus exhausted is rich with the desorbate, which corresponds to the component of the feed gas which has been adsorbed and is released upon reduction in the pressure. In certain known systems, the depressurized exhaust portion of the cycle is followed by introduction of a purge gas at the product outlet end of the adsorption. A new cycle is commenced with the introduction once again of pressurized feed gas after purging has been completed.
After carbon oxides and other impurities are removed from the shifted gas, in conventional processes, the H.sub.2 in the shifted gas is reacted with N.sub.2 that has accompanied the H.sub.2 through the shift reactor. This is accomplished by compressing the H.sub.2 and N.sub.2 and feeding these gases into an ammonia synthesis unit where the H.sub.2 and N.sub.2 are converted into ammonia (3H.sub.2 +N.sub.2 .fwdarw.2NH.sub.3). As the yield of this ammonia synthesis reaction is typically considerably less than 100%, NH.sub.3 in the product stream exiting the ammonia synthesis unit is collected from the product stream by cooling and condensation, and at least a portion of the unreacted H.sub.2 and N.sub.2 in the product stream is recycled through the ammonia synthesis unit.
U.S. Pat. No. 4,572,829 discloses a process in which a reformed gas mixture to be employed for ammonia synthesis is purified, following shift conversion, by the selective catalytic oxidation of residual carbon monoxide and the selective adsorption of carbon dioxide and water so as to render unnecessary the methanation of carbon oxides.
U.S. Pat. No. 4,592,860 discloses a process and apparatus for ammonia synthesis gas production using a PSA system.
U.S. Pat. No. 4,725,380 discloses the production of ammonia synthesis gas by partial oxidation of a hydrocarbon feed stock, using air, oxygen enriched air, or oxygen depleted air, in admixture with steam, followed by shift and the removal of the excess of nitrogen, and also impurities such as carbon oxides and methane, by pressure swing adsorption.
U.S. Pat. No. 5,252,609 discloses synthesis gas production comprising primary catalytic steam reforming a first stream of desulfurized hydrocarbon feed stock, optionally followed by secondary reforming using an oxygen-containing gas, and then cooling; adiabatically low temperature steam reforming a second stream of the feed stock, preferably adding a hydrogen-containing gas, and then subjecting the product to partial oxidation with an oxygen-containing gas, and then cooling; and mixing the cooled products.
U.S. Pat. No. 5,254,368 discloses a periodic chemical processing system with a single-bed rapid cycle PSA device.
The disclosure of each of the above U.S. patents is incorporated herein by reference in its entirety.
Conventional processes for ammonia synthesis rely on the regulation of the feed rate of only a few, fixed composition, feedstock components. However, these conventional processes are not designed to operate with a heterogeneous feedstock. The term "heterogeneous feedstock" as used herein refers to a non-homogeneous carbon-containing feedstock, containing a mixture of dissimilar feedstock components, in which the composition of the feedstock can vary widely over time as the result of variations in the composition of one or more of the feedstock components and/or variations in the relative amounts of the components in the feedstock.
There is a need for a process for making ammonia from a heterogeneous feedstock.